Discharged domestic wastewater often contains phosphorus, in the form of dissolved phosphate, arising from household detergents, etc. Dissolved phosphate is a problem because, in a body of open water (e.g a lake), it can lead to blooms of algae, to the detriment of other life-forms.
In some cases, the limitation on the number of dwellings that can be permitted around a lake is defined by the phosphorus in the effluents from the dwellings. It is becoming common for authorities to impose levels for P-content, in discharged water, typically at 1 mg/liter, or less. Indeed, maximum permitted levels of 0.3 mg/liter are becoming standard.
As is well-known, one way by which dissolved phosphate can be taken out of solution in wastewater is by adsorption of the phosphate onto a suitable sorbing medium. The present invention is not concerned with adsorption, but with taking the phosphorus out of the water by mineral precipitation reactions. In the invention, the aim is to convert the dissolved phosphate, by chemical reaction, into an insoluble solid, which precipitates.
It has been conventional, in some municipal sewage treatment plants, to address the problem of an excessive P content by dumping bags of alum into the water. The alum serves as a source of aluminum sulphate. Alum is very soluble. When the alum enters the water, Al3+ and SO42xe2x88x92 ions quickly enter solution. The dissolved Al3+ ions combine with any phosphate PO43xe2x88x92 ions that might be present, to form aluminum phosphate. Under the conditions of (approximately) neutral pH likely to be encountered in a sewage treatment system, the aluminum phosphate is insoluble, and precipitates. The precipitant may comprise the mineral variscite, AlPO4.2H2O.
The use of alum can be effective to drive down the phosphate-P content to 1 mg/liter, or less. However, one problem with the use of alum is that dissolved Al3+ ions do not remain in solution for long. The Al3+ ions have an affinity for phosphate, and so the phosphate PO43xe2x88x92 ions that happen to lie close to the point at which the alum enters the water react with the dissolved aluminum Al3+ ions, as desired, to form aluminum phosphate, which precipitates; however, the dissolved Al3+ ions from the sulphate which do not immediately pick up phosphate PO43xe2x88x92 ions, soon tend to react with the water, and to form aluminum hydroxide. Aluminum hydroxide, Al(OH)3, like the aluminum phosphate, is also insoluble at normally-encountered pH levels, so the hydroxide, too, precipitates, generally in the form of the mineral gibbsite.
The problem with putting alum into the water, to take out the phosphate, is that alum is very soluble, and the alum dissolves too quickly; but only a few of the many Al3+ ions that enter the water actually reach, and react with, the phosphate ions to precipitate as aluminum phosphate; the remainder of the large quantity of aluminum ions that enter solution precipitate out as aluminum hydroxide (gibbsite). Because so many of the Al3+ ions from the alum precipitate as the hydroxide before they can react with a phosphate ion, a large excess of alum is generally needed, in order to draw a given concentration of phosphate out of solution, by precipitating aluminum phosphate.
Thus, the technique of treating water contaminated with phosphorus by adding alum to the water results in the unwanted precipitation of large quantities of gibbsite (aluminum hydroxide). Besides, adding alum is labour-intensive; because alum dissolves quickly, the bags have to be added one at a time at regular intervals, rather than just once as a large volume that would last for a long period.
In a municipal sewage plant, at least the inconvenience associated with the use of alum can be managed, and it is conventional, in municipal plants, to treat phosphorus by adding alum to the water. But when it comes to domestic water treatment systems, e.g single-dwelling septic-tank systems, of course it is out of the question, as a practical system for treating excess phosphorus, to expect householders, every few days, to add a bag of alum into the water emerging from the septic-tank.
Gibbsite is (almost) insoluble, and, in the municipal systems, the large volumes of excess gibbsite precipitate as a flocculant in the sewage treatment. This adds to the sludge produced by the plant, which is a nuisance. It is recognised that, because alum is so soluble, though suitable as a treatment material for use in municipal systems, is only marginally suitable for use in domestic systems.
On the other hand, the ion-association reaction, in which dissolved phosphate PO43xe2x88x92 combines with dissolved aluminum Al3+ to form aluminum phosphate, is indeed an effective reaction for getting rid of the phosphate. The aluminum reaction is advantageous because aluminum phosphate is highly insoluble, and precipitates out of the water rapidly. The concentration of dissolved phosphate in the water can easily be reduced to 1 mg/liter, or less, when this reaction is able to take place properly and fully.
The invention is aimed at focussing the aluminum ion-precipitation reaction more efficiently onto the phosphate, whereby the phosphate can be precipitated, without the unwanted precipitation of large excess quantities of other unwanted minerals.
It is recognised that the ion-exchange reaction can be made to work with metals other than aluminum, but the aluminum reaction is the most economically practicable, and the invention is described herein as it relates to the aluminum reaction.
The invention is aimed at treating phosphorus by providing conditions for the ion-exchange reaction to take place, economically, and in a manner that requires little by way of on-going attention or maintenance. In particular, it is an aim of the invention to provide a phosphorus treatment system which requires no more attention than a conventional septic tank system. To be acceptable in a domestic water treatment context, a phosphorus treatment system must not require the householder to add a bag of treatment material into the water every few days; nor must it require the householder to clean out accumulated deposits of precipitated minerals. For the system to be acceptable, the service and maintenance demands should be compatible with those of a domestic septic tank water treatment system, i.e no more than once every year or two.
This is not to say that the invention is restricted only to domestic wastewater systems. It is recognised that the invention is suitable for use generally, in cases where the wastewater containing the dissolved phosphorus also contains dissolved ammonium.
It is known that the solubility of aluminum hydroxide (gibbsite) increases as the pH of the water decreases. For example, a typical septic-tank wastewater effluent, when at a neutral pH, has a solubility of aluminum hydroxide of only about 0.01 milligrams per liter; whereas, once the pH drops below about 5.5, the solubility of aluminum hydroxide in otherwise the same water increases sharply. The solubility of aluminum hydroxide reaches 1 mg/l at a pH of about 4.8, and 10 mg/l at a pH of about 4.5.
The invention involves creating the conditions whereby the pH of the phosphorus-contaminated water is lower than about 5.5, and preferably lower than about 5. It is recognised, in the invention, that when the pH has dropped to this low value, now the source of the Al3+ ions needed for the formation and precipitation of aluminum phosphate can be aluminum hydroxide (gibbsite). Thus, instead of gibbsite being the insoluble substance that, unfortunately, precipitates in large quantities following the introduction of excessive quantities of aluminum sulphate (alum) into solution, now, at the low pH, gibbsite is soluble enough, itself, to serve as the source of the Al3+ ions. As the water becomes acidic, more gibbsite dissolves, and more Al3+ ions go into solution; if phosphate is present in the water, the Al3+ ions have an affinity for the phosphate, and aluminum phosphate precipitates.
If the pH does not fall far enough, not enough Al3+ ions (from the gibbsite) will enter solution, and there will not be enough dissolved Al3+ ions to deal with all the dissolved phosphate. It is, however, possible for the pH to fall too far, whereby too much of the gibbsite would dissolve; in that case, the result would be that the phosphate would be dealt with very effectively, but now there would be an excess of Al3+ ions in solution, because not all the Al3+ ions would be taken up by the phosphate PO43xe2x88x92 ions, and be precipitated as aluminum phosphate. Thus, if the pH drops too far, too much Al3+ will remain in solution. (The excess Al3+ in solution will eventually precipitate out, once again as gibbsite, if and when the pH of the water later becomes more neutral). Theoretically, there is a level of pH at which the concentration of Al3+ that enters solution is just enough to precipitate all the phosphate. It is recognised that this level is around pH=5.5. It is also recognised that it is better to err on the side of too much Al3+ than too little, in that an excess of aluminum in the water is an inconvenience, whereas an excess of phosphate is a contaminant.
When the pH was neutral, the solubility of aluminum hydroxide was very low; therefore, any Al3+ ion that did not immediately react with a phosphate ion would not remain in solution, but would precipitate out as aluminum hydroxide. But when the pH is low, the solubility of aluminum hydroxide being now high, now many of the Al3+ ions that do not immediately react with phosphate ions can remain in solution until a phosphate ion becomes available.
Ideally, just enough Al3+ ions should be released into solution as will deal with all the phosphate ions (i.e as will precipitate all the phosphate as aluminum phosphate). If not enough Al3+ is present, not all the phosphate will be dealt with; if too much Al3+ is present, the excess will remain in solution while the pH remains low, but will start to precipitate out, as gibbsite, when the pH rises.
It is recognised that the pH should not be driven too lowxe2x80x94that is to say, below about 4.5xe2x80x94because at that very low pH, the solubility of aluminum hydroxide (gibbsite) now exceeds 20 milligrams per liter, which is far more than can be used up dealing with the dissolved phosphate.
Effluent water from a septic tank system, if it is contaminated with phosphate, contains typically 10 mg/l of phosphate-phosphorus. The water will usually have been at roughly neutral pH when picking up the phosphate, and while passing through the septic tank system, and the solubility of the phosphate-producing substances is around 10 mg/l at neutral pH.
The solubility of aluminum phosphate in fact goes down as the pH goes down, reaching a minimum of about 0.02 mg/liter at a pH of about 4.7. Thus, as the pH drops, the solubility of aluminum hydroxide increases, whereby more Al3+ ions are available in solution to react with (i.e to cause precipitation of) the dissolved phosphate; and at the same time, the solubility of aluminum phosphate decreases, whereby aluminum phosphate is urged even more strongly out of solution and into precipitation.
If effluent water from a septic tank system, contaminated with 10 mg/l of phosphate-P, is driven down to a low pH, and is then passed over or through a body of aluminum hydroxide, in the manner as described herein, it can be expected that the phosphate-P content will drop to below 1 mg/l. Some jurisdictions require the water to contain no more than 0.3 mg/l, and the invention is capable of enabling even this degree of remediation to be attained, if the natural conditions are favourable and if the engineered conditions are done carefully and properly.
It should be noted that the body of aluminum hydroxide (gibbsite) does not all quickly dissolve. Thus, a large body of gibbsite can be provided, at first, and that body will remain in place, as a body, for a long period. Provided the pH does not fall too low, the amount of aluminum hydroxide needed to saturate the water, even though the solubility thereof is greater than it was at neutral pH, is still small, whereby a body of aluminum hydroxide will last a long time before it dissolves away. (This may be contrasted with the aluminum sulphate treatment system: the sulphate was so soluble that a large bagful dumped in the water would be completely dissolved in a few hours.)
One requirement of the invention, as mentioned, is that the pH of the water must be driven down to below about 5.5. In a sewage treatment system (domestic or municipal), ammonium is oxidized to nitrate; it is recognised that, at least in some types of effluent water, the pH can be driven down to below 5.5 simply be ensuring that the normal process of oxidation of the ammonium is fully completed. The normal oxidation reaction is:
NH4++2O2xe2x86x92NO3xe2x88x92+2H++H2O
Generally, in sewage systems, it does not matter if a few percent of the ammonium has not been oxidized into nitrate, when the water is discharged from the treatment system. It will (usually) oxidize naturally later on. However, sometimes, complete oxidation of all the ammonium does take place, and it is known, in such instances, that the pH suddenly starts to drop, as the last few percent of the ammonium are consumed. Then, effluent water that has undergone a complete oxidation of ammonium has been observed to have a pH as low as 4.5 or even 4.
Oxidation of the ammonium takes place in the aerobic station of the water treatment system. Of course, complete oxidation can be procured simply by making the aerobic station large enough, and efficient enough. However, as mentioned, usually the designers of a sewage treatment plant (municipal or domestic) are not concerned to remove every last molecule of ammonium, and aerobic treatment stations, generally, are not good enough to achieve the degree of completeness of oxidation that is required to drive the pH down to the levels at which gibbsite can be used as the source of the aluminum to precipitate the phosphate. Of course, some (perhaps over-engineered) aerobic treatment stations in the past have been such as to cause complete oxidation of the ammonium, and in such cases, it has been observed that the pH does indeed drop to the low values, as described.
Gibbsite is a naturally occurring substance in many types of soil, and it has been noted that, when the acidity of a body of water reaches the 5 or 4.5 area (which can happen, for example, in a lake via the xe2x80x9cacid rainxe2x80x9d mechanism), the concentration of dissolved gibbsite does increase markedly. One of the reasons acidity is damaging to lake life is that the dissolved gibbsite tends later to precipitate (fatally) on the gills of fish.
Thus, it is known (a) that the pH of sewage treatment water tends to fall sharply when and if the ammonium undergoes more or less complete oxidation, and (b) that the solubility of gibbsite increases as the pH falls. What has not previously been understood is that these facts can lead to a practicable, economical treatment system for alleviating phosphorus from sewage water, being a water treatment system which involves engineering the conditions in which complete oxidation is procured, and which at the same time involves providing a source of gibbsite and passing the completely oxidized water over or through the gibbsite.
In the invention, the phosphorus-containing water that has been fully oxidized, and is of low pH, is passed over or through a body of gibbsite. Of course, the body of gibbsite must be engineered to be porous and permeable to the passage of water therethrough. One way in which this can be done is by applying a coating of gibbsite to grains of sand, and using the coated sand as the permeable body. The water takes the aluminum hydroxide into solution as it passes over and between the coated grains. The lower the pH of the water passing through the coated sand grains, the greater the concentration of aluminum hydroxide in the water.
In fact, it may be noted that, if the pH of the water passing over the coated sand grains were not indeed low, very little of the aluminum hydroxide would pass into solution. In other words, the passing water will only take up so much of the aluminum hydroxide into solution as is enabled by the pH level of the passing water. This means that the grains of sand that are coated with aluminum hydroxide can serve as the sand that is to comprise the basis of the aerobic treatment station. As the water passes through the sand, so the oxidation of the ammonium proceeds: at first, the oxidation is incomplete, and very little of the aluminum hydroxide is taken up; then, as oxidation is completed, the pH drops, and more and more of the aluminum hydroxide on the grains is taken up, into solution.
It should also be noted that, sometimes, water being treated in a sewage treatment plant might contain minerals, such as limestone, which buffer the acidity. In these cases, the presence of the minerals might prevent the pH from dropping below neutral levels, even when the ammonium has been thoroughly oxidized. Thus, the invention is unsuitable for use in alleviating phosphorus from hard water, i.e from water which contains enough buffering minerals to prevent the pH from dropping down to the low levels as described.
In this connection, also, the designer of the treatment system should see to it that the aerobic station of the treatment system in which the oxidation is to take place, does not itself introduce buffering minerals. If the aerobic station uses sand/gravel, or the like, such sand/gravel must be free of limestone, etc. If the local sand at the site is not substantially limestone-free, that local sand is not suitable for use in a phosphorus alleviation plant of the kind as described herein.